1. Field of the Invention
The invention relates to fixtures for discharge-type electric lamps. In particular, the invention relates to an apparatus and method for converting a mercury lamp fixture to a low pressure sodium lamp fixture.
2. Description of the Prior Art
Approximately 25 million mercury vapor lamps are presently in use. A large percentage of these lamps, especially those utilized as street lights, are mounted in elongated fixtures having a substantially hemispherical reflector enclosing the lamp and having a light-dispensing lens covering the reflector. The mercury vapor lamps inherently have a relatively low light efficiency on the order of 50 lumens per watt. As the conservation of all forms of energy, electrical energy in particular, becomes increasingly important, it has become essential to find a higher efficiency light source for streetlights, industrial lights, quarry and mine lights, and similar applications. In addition to the relatively low efficiency of the mercury vapor lamps, recently it has been suggested that such lamps, when damaged, emit ultraviolet radiation which may be a potential cause of skin cancer, cataracts and temporary blindness.
It is well known that low pressure sodium lamps offer a number of inherent advantages over mercury vapor lamps. For example, low pressure sodium lamps have a light efficiency on the order of 180 lumens per watt which is the highest efficiency of any commercially available lamp. The sodium lamps produce a spectral light output closely corresponding to the peak response of the human eye thereby creating improved nighttime visibility. Furthermore, such lamps have no ultraviolet output and are capable of operation in virtually any weather conditions. Other well-known features of low pressure sodium lamps include their high performance in fog and mist conditions, their nonpolluting characteristics and their low operating temperatures.
Previous attempts to convert mercury lamp fixtures to low pressure sodium lamp fixtures involved either the replacement of the entire fixture or the replacement of substantially all of the internal parts of the fixture, including the reflector. Since the rising cost of electrical energy, in combination with the dramatic increase in efficiency of modern generation low pressure sodium lamps, has made it economically feasible to convert existing mercury lamp fixtures to low pressure sodium lamp fixtures, it becomes an object of the invention to provide an apparatus and method for making such conversions with a minimum alteration in the existing fixtures and with the additional benefits of reduced health risks and optimum spectral light output.